1. Field of the Invention
The present invention relates to a microscopic positioning device using piezoelectric elements and a tool position and orientation-compensating method using the microscopic positioning device.
2. Description of the Related Art
In a machine tool, when the blade edge of a tool is deviated from the intended position and orientation, the deviation amount directly affects the accuracy of form of a processed object. Especially when performing the ultra-precision machining of a free-form surface, even a micron-order error of the blade edge cannot be neglected as a machining error. In general, the position and orientation of the tool are compensated by an adjustment mechanism, such as a micrometer, after estimating a necessary compensation amount through a method for inferring how much and in what direction the tool is deviated on the basis of an actual machining result and a method for measuring distance between a benchmark and the tool blade edge by using a microscope and a displacement sensor.
Although the micrometer is capable of making a micron-order error adjustment, the improvement of accuracy of measuring devices made it possible to estimate the deviation amount of the tool to nano-order resolution, based on machining results of the work. Therefore, it is desired that the tool position and orientation be compensated by the nano-order adjustment. In general, a piezoelectric element, which expands according to the level of an applied voltage, is commonly utilized as an actuator for positioning a micro-stroke of several to several dozen microns in the nano-order.
Known as such a device with high accuracy is the one in which a spindle unit is fixed through an RCC (remote center compliance) mechanism to a Z-axis table that is driven in a Z-axis direction by a screw delivery mechanism. In the RCC mechanism, four piezoelectric elements are arranged in a circumferential direction. By expanding/shrinking these piezoelectric elements, the inclination of the spindle unit is adjusted, to thereby accurately control the position and orientation of the tool (JP 7-299700A).
In order to achieve one nanometer resolution in a rotating motor and a linear motor, a precision and complicated machine configuration is required. As for the piezoelectric element, on the contrary, as long as the applied voltage is controlled, one nanometer resolution can be achieved relatively easily. For instance, in the case of a piezoelectric element which expands by 10 microns in response to a voltage of 100 volts, one nanometer resolution can be achieved by changing the voltage with 10-millivolt resolution. Consequently, a position adjustment in nanometer unit is made without difficulty.
Depending on the machining, there are differences in directions in which the compensation of the tool position and orientation has to be made. In consideration of every machining, however, there needs to be a mechanism capable of carrying out an orthogonal three-axis and rotational three-axis six-degree-of-freedom adjustment to arbitrarily compensate both the position and orientation of the tool. However, since a one-degree-of-freedom configuration can be considered to form a single spring system, six degrees of freedom are accordingly considered to comprise six springs jointed in series. Such a multidegree-of-freedom configuration is then weak in machine rigidity. There is no point in compensating the tool position and orientation in the nano-order if the machine configuration becomes weak. Therefore, the securement of machine rigidity is important.
Although the piezoelectric element is highly resistant to an external force acting in a compressing direction, it is easily affected by an external force acting in an expanding direction. The piezoelectric element that is commonly utilized is a laminated one with a configuration in which a large number of thin elements having a piezoelectric effect are superimposed on one another. The elements are simply united to one another through thin adhesive layers. Therefore, if the elements are applied with a force acting in a pulling direction, the adhesive layers come unstuck, resulting in damage. In other words, if the piezoelectric elements are incorporated into a structural body, the structural body becomes weak in rigidity depending on directions in which the piezoelectric elements are arranged.
The above problem can be solved if only the configuration is formed such that the external force constantly acts in a direction of compressing the piezoelectric elements. For instance, in the case of a positioning device having a movable part for performing a relative displacement with respect to a base and positioning the movable part, two piezoelectric elements are utilized, and one-side ends of both piezoelectric elements are fixed onto two respective opposite faces of the movable part into alignment, while the other-side ends of the piezoelectric elements are each fixed to the base. With such a configuration, even if the movable part is applied with the external force acting in the direction of expanding one of the piezoelectric elements, the force acts in the direction of compressing the other piezoelectric element without fail. For this reason, the piezoelectric element which is compressed can resist the external force with high rigidity.
The above-mentioned configuration, however, has a problem. Because of its polarity, the piezoelectric element is basically displaced only in the expanding direction. Therefore, if both ends of each of the piezoelectric elements are fixed as described above in a state where no voltage is applied to the piezoelectric elements, one of the piezoelectric elements must be displaced in the compressing direction, while the other in the expanding direction, in order to displace the movable part. However, the piezoelectric element cannot be displaced in the compressing direction. To solve this problem, if both the ends are fixed in the same manner taking the displacement of half the maximum displacement amount as a neutral point, the movable part can be displaced by expanding/shrinking the two piezoelectric elements in opposite directions. If the applied voltage is turned to 0 volt, however, the piezoelectric elements are both compressed, and the piezoelectric elements themselves are destroyed, so that the power source is required not to be turned off. For this reason, the configuration in which two piezoelectric elements are arranged in the movable part to be opposed to each other as described above has not been applied in prior art.